The 1 core per game is a general rule of thumb, and usually for the lower numbers. i.e. 4 cores and 4 games. Once you start going above 8 games it is usually less of a requirement. Most of the rule of thumbs are going to be for "normal" team sizes, and not mass multiboxing teams, where it gets more flexible.

As for your crashes. The easy test would be, do you have the same issue when running 10 clients, rather than 12. If not, and the vram pressure is lessened (i.e. down to about 6.5GB by my rough calc), then the most likely cause is going to be the lack of vram.

The error is a hardware issue, so whether it is overload or actual faulty device, only further testing will tell.